Allow binding of functions whose return types are convertible to R, fixes 9

Author: TartanLlama

function_ref.hpp

@@ -115,7 +115,7 @@ struct is_invocable_r_impl : std::false_type {};
 
 template <class R, class F, class... Args>
 struct is_invocable_r_impl<
-    typename std::is_same<invoke_result_t<F, Args...>, R>::type, R, F, Args...>
+    typename std::is_convertible<invoke_result_t<F, Args...>, R>::type, R, F, Args...>
     : std::true_type {};
 
 template <class R, class F, class... Args>
@@ -152,7 +152,7 @@ template <class R, class... Args> class function_ref<R(Args...)> {
                 detail::is_invocable_r<R, F &&, Args...>::value> * = nullptr>
   TL_FUNCTION_REF_11_CONSTEXPR function_ref(F &&f) noexcept
       : obj_(const_cast<void*>(reinterpret_cast<const void *>(std::addressof(f)))) {
-    callback_ = [](void *obj, Args... args) {
+    callback_ = [](void *obj, Args... args) -> R {
       return detail::invoke(
           *reinterpret_cast<typename std::add_pointer<F>::type>(obj),
           std::forward<Args>(args)...);

tests/issues.cpp

@@ -5,4 +5,18 @@ TEST_CASE("Issue #2") {
 	const auto lam = [](int ) {};
 	tl::function_ref<void(int)> ref = lam;
 	(void)ref;
+}
+
+struct Fruit {};
+struct Apple : Fruit {};
+
+Apple* getApple() { return nullptr; }
+
+tl::function_ref<Fruit* ()> bar()
+{
+  return getApple;
+}
+
+TEST_CASE("Issue #9") {
+  REQUIRE(bar()() == nullptr);
 }